[llvm-dev] Clang 5, UBsan, runtime error: addition of unsigned offset to X overflowed to Y

[Jeffrey Walton via llvm-dev]

We have code that processes a buffer in the forward or backwards
direction. It looks similar to the following
(https://github.com/weidai11/cryptopp/blob/master/adv-simd.h#L1138):

uint8_t * ptr = ...
size_t len = ...
size_t inc = 16;

if (flags & REVERSE_DIRECTION)
{
    ptr += len - inc;
    inc = 0-inc;
}

while (len > 16)
{
    // process blocks

    ptr += inc;
    len -= 16;
}

Clang 5.0 and UBsan is producing findings
(https://travis-ci.org/noloader/cryptopp/jobs/317442299#L967):

adv-simd.h:1138:26: runtime error: addition of unsigned offset to
0x000003f78cf0 overflowed to 0x000003f78ce0
adv-simd.h:1140:26: runtime error: addition of unsigned offset to
0x000003f78ce0 overflowed to 0x000003f78cd0
adv-simd.h:1142:26: runtime error: addition of unsigned offset to
0x000003f78cd0 overflowed to 0x000003f78cc0
...

Lines 1138, 1140, 1142 (and friends) are the increment, which may
stride backwards:

    ptr += inc;

The overflow is troubling because signed integer overflow is undefined
behavior. The code above depends on unsigned wrap, and that's well
defined behavior. We don't know where the signed operations are coming
from.

Previous versions of Clang and GCC did not produce a finding.

My questions are, what is going on, and how do I fix it?

Jeff